Adobe After Effects CS3 - Manual (Page 257)

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Chapter 10: Masks, transparency, and
keying
Transparency overview
Use masks, keying effects, and mattes to determine which parts of a layer are transparent, allowing other layers to
show through.
About transparency
Before you can create a composite from multiple images, parts of one or more of the images must be transparent. You
can use alpha channels, masks, mattes, or keying to define which parts of an image are transparent and which parts
of an image can be used to obscure parts of another image. By manipulating transparency and choosing blending
modes, you can create a variety of visual effects.
About alpha channels and mattes
Color information is contained in three channels: red, green, and blue. In addition, an image can include an invisible
fourth channel, called an alpha channel, that contains transparency information.
Channels at a glance
A. Separated color channels B. Alpha channel C. All channels viewed together
An alpha channel provides a way to store images and their transparency information in a single file without
disturbing the color channels.
When you view an alpha channel in the After Effects Composition panel or an Adobe Premiere Pro Monitor panel,
white indicates complete opacity, black indicates complete transparency, and shades of gray indicate partial trans-
parency.
A
B C